The double-hung type of windows that have been used for many years present a special security problem if the windows are to be left partially open for ventilation purposes. Most of the lock systems initially or previously provided for windows of the foregoing type are now inoperative by reason of long continued use or misuse, or such lock systems were never adequate to prevent unwanted intrusions if the windows were not completely closed and locked. While it is acknowledged that some previous lock attachments have been provided to hold one or both sashes of a double-hung window in closed or open positions, such previous locking devices have usually been operatively positioned between a window sash and the window frame. With such devices installation is relatively complicated.